Heavy Ions Theory Review *

The advent of collider energies to heavy–ion physics is leading to a profound change in the field. The new tools available, specially the access to the large transverse momentum part of the spectrum, allow for an unprecedented characterization of the high–density state created in such collisions. Accordingly, the traditional goal of producing a Quark-Gluon Plasma, the deconfined state of quarks and gluons predicted from QCD, has been enlarged to the study of several other mechanisms as thermalization, parton distribution functions at very small-x or in–medium evolution of parton showers. In general words, a new line is emerging which attempt to study how the collective properties of the fundamental interactions appear. For that end, large energy densities in extended regions need to be produced in contrast with the traditional direction in high–energy physics which attempt to create the largest possible energy scales in well localized spacial regions for new physics to become observable. The relevant questions which can be addressed in experiments of heavy ion collisions can be (artificially) classified depending on the time scale of the relevant phenomena: i) before the collision, the structure of the two Lorenz-contracted nuclei is rather different from a typical hadron at the